The present invention relates to an improved welding or brazing apparatus and to an improved method of welding or brazing. In particular, the invention relates to an improved method of plasma welding or brazing, more particularly plasma arc welding, and to an improved welding apparatus suitable for carrying out this method.
The plasma arc welding process, which is a particular type of Tungsten-Inert Gas Welding, has been known for several years and is widely used industrially, in particular in the automotive industry. The process has been used in particular for welding together steel components or workpieces and is also increasingly being used to weld aluminium.
Plasma arc welding uses a non-consumable electrode, typically of tungsten, fitted to a welding torch. The torch comprises a nozzle which creates a gas chamber surrounding the electrode. The arc which is generated between the electrode and the workpiece is a xe2x80x9ctransferred arcxe2x80x9d and heats inert gas fed into the chamber to a temperature where it becomes ionized and conducts electricity, ie it becomes a plasma. By the term xe2x80x9ctransferred arcxe2x80x9d is meant an arc which transfers from the electrode to the workpiece, so that the workpiece is part of the electrical circuit. The electrode is recessed within the nozzle so that it is not possible for it to touch the workpiece; this greatly reduces the possibility of contaminating the weld with electrode material.
In conventional arc welding, the arc is generated between a non-consumable electrode and the workpiece by establishing a voltage differential between the electrode and the workpiece, touching the electrode against the workpiece to initiate the arc and then withdrawing the electrode once the arc is initiated.
Since, in plasma arc welding, the recessed electrode cannot be touched against the workpiece in order to start the arc, it is first necessary to ignite a low-current pilot arc between the electrode and the nozzle. The pilot arc is a non-transferred arc and is generally generated by the use of high frequency AC power or by a high voltage d.c. pulse, which helps to break down the arc gap and ionize the gas flowing through the orifice of the nozzle, so that it will conduct the pilot arc current. By the term xe2x80x9cnon-transferred arcxe2x80x9d is meant an arc which is established and maintained between the electrode and the nozzle, so that the workpiece is not in the electrical circuit. After initiation of the arc, a d.c. pilot arc is maintained.
In the plasma arc welding of steel workpieces, the workpiece is generally maintained at a positive voltage and the electrode at a negative voltage. This is beneficial since most of the heat required for welding is generated at the positive component, so that this polarity improves weld quality and increases electrode life.
In order to maintain a robust and effective pilot arc, it is necessary to ensure that the electrode is in good condition and does not deteriorate through ageing. If the electrode does age, and the pilot arc becomes weak, it is necessary to stop the process and change the electrode. This can be very costly when it involves stopping a production line for the time taken to change the electrode. In order to avoid this, it is standard practice to change electrodes before they age, at a time which is convenient, for example at the end of each shift, even if the electrode is not showing any signs of ageing. This has the disadvantage that electrodes are changed more frequently than is necessary, which leads to increased costs.
If the plasma arc welding of aluminium workpieces is being carried out, and the workpiece is maintained at a positive voltage and the electrode is maintained negative, then problems may arise because of the formation of an aluminium oxide coating on the surface of the aluminium. In order to remove this oxide coating, which inhibits the formation of a high quality weld, it has been found advantageous to vary the polarity of the electrode and the workpiece. The electrode is still maintained negative for most of the welding time, but the polarity is reversed during the welding, so that for typically 20 to 30% of the time, the electrode is positive.
It is a disadvantage of this process that the d.c. pilot arc, which is maintained throughout the welding, is superimposing a d.c. current on the variable polarity welding current. The d.c. current is flowing from a negative electrode to a positive workpiece, so that during the periods of reverse polarity the reverse current must be increased to compensate for the d.c. current.
The undesirably high positive polarity, applied during this reverse current period, leads to increased deterioration of the electrode and hence to an increase in the frequency with which the electrode must be replaced.
It is an object of the present invention to provide a welding or brazing process in which the above disadvantages are reduced or substantially obviated. It is a further object of the invention to provide a welding or brazing apparatus to carry out this process.
The present invention provides a welding or brazing process in which a direct current pilot arc is maintained throughout the process, characterised in that the process comprises the steps of setting a pilot arc power supply to a relatively high level, generating a high voltage spark discharge between a non-consumable electrode of a plasma torch and its attendant constricting orifice in order to start the flow of a pilot arc current, detecting the pilot arc current and reducing the pilot arc current to a relatively low level.
The invention further provides an apparatus for carrying out welding or brazing, comprising a welding or brazing torch, a controller and a power supply, characterised in that the controller further comprises means for detecting the flow of a pilot arc current and means for reducing this current on detection.
While the process according to the invention is particularly suitable for use in a plasma arc welding operation, it is also suitable for use in other welding, brazing and soldering operations, including plasma arc and plasma spot welding processes, which may be carried out with or without the use of a homogenous filler material, and brazing processes using a heterogeneous filler material, which may for example be a bronze based filler material.
In a preferred embodiment of the process according to the invention, the pilot arc current is maintained at the relatively low level, until welding is to be undertaken, the welding torch is positioned at the workpiece when welding is to occur, the pilot arc current is increased to a relatively high level, a welding arc is struck between the torch and the workpiece, the current flow of the welding arc is detected and the pilot arc current is reduced to the relatively low level.
In a particular preferred embodiment of the process according to the invention, the pilot arc current is increased as rapidly as possible to a predetermined relatively high level and reduced to the relatively low level once the welding arc is detected.
In an alternative particularly preferred embodiment of the process according to the invention, the pilot arc current is increased gradually towards a predetermined relatively high level, but is reduced to the relatively low level as soon as the welding arc is detected.
It is an advantage of the process according to the invention that it provides for improved reliability of starting the pilot arc under conditions where the electrode is subject to wear or erosion or of a design or material not conducive to starting of the pilot arc at a low current level. Reduction in the level of pilot arc current once the pilot arc is established reduces the thermal loading on the torch.
Where the process is carried out according to the preferred embodiment described above, in which the pilot arc is increased to a relatively high level before each welding operation, this improves the ability of the welding arc to transfer between the electrode and the workpiece.